1. Field of the Invention
The present invention relates to an optical transmitter for use in an optical fiber communication system and, more particularly, to an improvement in an optical transmitter to which DPSH-IM (Direct Phase-shift and Self-Homodyne Intensity Modulation) is applied.
In optical fiber communication systems, the modulation speed has been increased. Wavelength chirping is a problem in the direct intensity modulation of a laser diode. The chirping causes pulse distortions when a pulse passes through an optical fiber with wavelength dispersion. From the viewpoint of fiber losses, the most desirable wavelength is 1.55 .mu.m. At this wavelength, a normal fiber has an approximate wavelength dispersion of 17 ps/km/nm. This limits the transmission distance. To avoid this problem, the use of an external modulator has been proposed. However, there is a problem with the external modulator at present that a high driving voltage is required for it.
To overcome the above described difficulties in high-speed modulation systems, i.e., problems of chirping and high driving voltage, and to achieve a narrowed light spectrum and a reduced driving voltage, we proposed DPSH-IM as a new intensity modulation system (PCT/JP89/00220 and IOOC '89, 20D4-5) and have proceeded with experimental and theoretical investigations of it.
2. Description of the Related Art
In the DPSH-IM system, modulation current pulses with a small amplitude are given to a laser diode being applied with a bias current higher than the threshold current to thereby obtain phase-modulated oscillating light. The phase-modulated light is passed through an optical interferometer to be converted into intensity-modulated light. Details of its operating principle is given in Shirasaki, M., Nishimoto, H., Okiyama, T., and Touge, T.: Fibre transmission properties of optical pulses produced through direct phase modulation of DFB laser diode, ELECTRONICS LETTERS 14th Apr. 1988 Vol. 24 No. 8 pp. 486-488.
According to the DPSH-IM, since small-amplitude modulation is possible with the use of a laser diode applied with a high bias current, it becomes possible to structure a system which is not easily affected by chirping and operates with a low driving voltage. However, in order to achieve still higher modulation speed and/or still longer transmission distance, it is required that the waveform deterioration due to wavelength dispersion is further reduced.